Field of the Invention
Embodiments of the invention generally relate to zeolite compositions and methods for forming such zeolites, and more particularly to zeolite with desirable crystal habits and methods for tailoring such crystal habits.
Description of the Related Art
The global synthetic zeolite industry accounts for approximately $1.9 billion in annually revenue. Currently, the largest market for zeolites is that of catalysts for petroleum refining; however, there is an increased demand for zeolites in diesel emissions reduction. Moreover, zeolite properties including acidity and nanoporosity are useful for ion exchange and selective separation applications. Research in both industry and academia seek to design more rational synthetic approaches capable of improving zeolite properties for existing petrochemical processes, for emerging applications, and for research objectives aimed to expand zeolite application to non-conventional markets, such as sensor technologies, drug delivery, and enantioselective catalysis, and separations.
The shape-selectivity of zeolites can be exploited for commercial applications in catalysis, ion exchange, and separations by the judicious selection of crystal structures with nanopore geometries commensurate with sorbate molecules. Zeolites tend to form anisotropic crystals with pore openings presented on low surface area faces and channels oriented axially along the longest crystal dimensions, which limit sorbate molecule access to pores on exterior crystal surfaces and increase the internal path length for molecular diffusion. These factors impose severe mass transport limitations that reduce molecular flux and decrease the yield, selectivity, and/or lifetime of zeolite catalysts, which poses a pervasive challenge to optimize zeolite crystal habit. As such, a strategic aim is to design more effective, facile, and inexpensive synthetic pathways to selectively tailor crystal habit with precise and predictive control.
There are several approaches that can be used to modify zeolite morphology. Adjustments to synthesis conditions, such as molar composition, solvent, pH, and temperature, have marginal effects on the kinetics and thermodynamics of crystal growth. Templates, such as surfactants, gels, and porous solids can be used to synthesize crystals with a morphology that mimics spatial features of the template. A third approach is the design of structure-directing agents (SDAs) capable forming specific crystal framework types. Studies of novel SDAs have elegantly demonstrated the synthesis of new crystal frameworks for the zeolite. However, the high costs associated with multistep SDA synthesis and the inability to recycle SDAs due to their thermal decomposition during zeolite post-treatment limits the commercial viability of this technique.
Changes in the synthesis techniques and composition can influence particle shape and size. Examples include the silica concentration, solution pH, the silica source (reagent selection), and the solvent. Process conditions, such as temperature or time of synthesis, can influence the overall size and size distribution of the zeolite crystal. Moreover, changes in the structure-directing agent (SDA) can have an impact on the type of formed crystal framework. Collectively, these approaches cannot achieve predictable control of zeolite crystal habit, particle size, surface structure, and other important properties utilized by the various fields. The use of modified SDAs or mixed SDAs is impractical for industrial applications due to the high cost of these reagents. Also, SDAs are specific to a single zeolite crystal framework or structure and are thus not universally applicable for all zeolite framework types. Additionally, SDAs usually become occluded within the zeolite structure and require additional process steps to remove the entrapped SDA.
Therefore, there is a need for a zeolite having a minimum crystal thickness along the diffusion pathways, such as with an aspect ratio of about 4 or greater, a zeolite with a maximum amount of active growth sites on the exterior surfaces, such as with a step density of about 25 steps/μm2 or greater, and methods for synthesizing such zeolites. Also, there is a need to have methods for synthesizing zeolites that are easily adaptable to the synthesis of multiple zeolite framework types, can be tailored for each framework type to selectively control zeolite crystal habit, particle size, and surface structure, are more robust, predictable, and efficient technique compared to traditional methods of altering synthesis composition and/or conditions, and are less expensive than traditional methods.